Functional and structural characterization of the insertion region in the ligand binding domain of the vitamin D nuclear receptor

Vitamin D nuclear receptor mediates the genomic actions of the active form of vitamin D, 1,25(OH)(2)D-3. This hormone is involved in calcium and phosphate metabolism and cell differentiation. Compared to other nuclear receptors, VDR presents a large insertion region at the N-terminal part of the ligand binding domain between helices H-1 and H-3, encoded by an additional exon. This region is poorly conserved in VDR in different species and is not well ordered as observed by secondary structure prediction. We engineered a VDR ligand binding domain mutant by removing this insertion region. Here we report its biochemical and biophysical characterization. The mutant protein exhibits the same ligand binding, dimerization with retinoid X receptor and transactivation properties as the wild-type VDR, suggesting that the insertion region does not affect these main functions. Solution studies by small angle X-ray scattering shows that the conformation in solution of the VDR mutant is similar to that observed in the crystal and that the insertion region in the VDR wild-type is not well ordered.